The genomes of the bacterium Helicobacter pylori and of its human host may determine who develops stomach cancer.

Studying people in two Colombian villages, researchers found that individuals from native populations who carried African strains of H. pylori were much more likely to develop stomach cancer than African descendants carrying an African strain.

The finding, published January 13 in the Proceedings of the National Academy of Sciences, suggests that when genes of the bacterium and its h...